The goal of this research is to develop and demonstrate means for producing orthographic ultrasonic transmission images of internal organs and tissues using technology that is entirely compatible with B-mode scanning. Although orthographic ultrasonic transmission imaging has a long history of development, only recently has its clinical potential begun to be realized. B-scanning, by contrast, has found extensive clinical application. One reason for transmission imaging's slow entry is its requirement for acoustic coupling to large areas on both sides of the body, restricting its potential use to those areas of the body for which water immersion is practical. Reflex Transmission Imaging (RTI) is a method for producing orthographic transmission images with augmented B-mode equipment. In this mode, transmission images in a plane normal to the beam are made by integrating the reverberations from beyond the focal zone of the transducer. These reverberations provide, in essence, a source of incoherent insonification. The transducer must be scanned in two directions, which may be done in any one of a variety of sector and translation combinations. In many applications transmission images should provide information that is complementary to B-scans. RTI will allow both to be made with the same instrument, and presented on the same display so that they can be easily cross-referenced. We envision that the ultrasongrapher would first perform a B-scan on the patient, using one of the scan directions of the PRI probe, then would select a plane of interest with a cursor in the B-scan display and initiate the two-dimensional RTI scan, which would take from 3 to 15 sec. A reflection-mode C-scan could be generated simultaneously, further extending the complement of interrelatable modalities. The basic concept of RTI has already been demonstrated at SRI International. We propose to continue basic studies of this imaging mode using the same computer-controlled research scanner with which the preliminary studies were performed, to detemrine the effects on image quality of aperture size, tissue inhomogeneity, and methods of signal processing, and to explore yet untried variations of the RTI process, including B-mode transmission imaging. We further propose to modify two B-mode instruments -- a dynamically focused peripheral anatomy scanner and a compound scanner -- with which we will conduct further research and preclinical in-vivo studies.